ZFS ashift tuning revisited

[Pawel Jakub Dawidek]

On Sat, Apr 20, 2013 at 06:20:58PM +0100, Steven Hartland wrote:
> From: "Alexander Motin" <mav at FreeBSD.org>
> > I've never really investigated the question, but I guess that increasing 
> > system-wide minimal desired ashift to 4K may cause additional disk space 
> > waste, possibly even more if compression is used. Though point that more 
> > and more disks are using 4K blocks is valid. From other side FusioIO 
> > periodically mention that they can effectively handle records smaller 
> > then 512b, so who knows what we seen in 10 years.
> 
> It does indeed increase waste a bit but currently I'd say thats a small
> downside compared to benefits of increased compatibility and performance
> on more mainstream / popular HW.

I saw presentation of one of the key ZFS devs, not sure which one (was
it Matt Ahrens?) stating that 4kB sectors can waste a lot of space for
reasonable small blocks and wide RAIDZ vdevs.

For example if you use RAIDZ2 and 8kB recordsize you will be able to use
only half of your pool size (effectively it becomes a mirror), because
of how RAIDZ allocates the space.

In my math half is a lot:)

PS. I'm adding zfs-devel@ to the thread.

-- 
Pawel Jakub Dawidek                       http://www.wheelsystems.com
FreeBSD committer                         http://www.FreeBSD.org
Am I Evil? Yes, I Am!                     http://mobter.com
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